This invention relates generally to semiconductor wafer processing and integrated circuit packaging. In particular, the invention relates to a selective underfill for opto-electronic and electromechanical bumped semiconductor wafers, flip chips and flip-chip assemblies, and a method for manufacturing a semiconductor wafer, flip chip or a flip-chip module with selective underfill.
Assembly of opto-electronic and electromechanical assemblies to printed wiring boards (PWB) is becoming increasingly important as discrete components are integrated to form cost-effective modules. Superior performance can be realized using flip-chip attachment technologies to drive small size. Since opto-electronic integrated circuits (ICs) contain light-emitting and light-detecting components that function between the die and the PWB, assembly technologies that block the light path to the light emitter or detector cannot be used. Flip-hip assembly technologies typically require underfill materials to bond the flip chip to the PWB, and these may block the light path. The underfill material structurally reinforces the solder bumps, mechanically adheres the flip chip to the PWB, and improves the reliability of the assembly.
Electromechanical devices such as surface acoustic wave (SAW) devices, micro-electro-mechanical system (MEMS) devices, integrated electromechanical devices, and other devices with movable parts may function in a degraded manner or not function at all if covered with an underfill material. These devices must remain free of underfill material when bumped and used in flip-chip assemblies.
In technologies of prior art, underfill materials are typically applied to the entire surface of the IC interface. In the liquid underfill dispense technique, the underfill is applied at the edges of the flip-chip bonded die and capillary action wicks the fluid under the die. During this process, the entire die surface is coated with the underfill. When using highly viscous, no-flow underfills, the underfill may be applied to the PWB prior to die placement. During solder reflow, the underfill liquifies and wets the entire die surface. In both cases, the underfill covers the entire die surface and interferes with light propagation between the die emitter and the die detector. If the underfill material is opaque and covers the optical elements, no radiation is transmitted. If transparent materials are used, defects such as bubbles, voids, particles or pockets of air next to the flip chip or printed wiring board may distort or inhibit the transmission of light. The transparent materials may degrade with time. Undue dispersion of light may occur with filler materials included within the underfill material for thermal coefficient of expansion matching. Electromechanical devices with mechanically or acoustically moving structures such as piezoelectric devices or surface-micromachined relays can have no underfill material covering the electromechanical element without impairing operation of the device.
An underfill material may be applied around the periphery of the flip-chip assembly and partially wicked into the interior region, leaving portions of the flip chip free of the underfill material, as described in U.S. Pat. No. 6,365,441, xe2x80x9cPartial Underfill for Flip Chip Electronic Packagesxe2x80x9d issued Apr. 2, 2002. As described in US published application US 2002/0037138, xe2x80x9cOptical Module and Method for Manufacturing Samexe2x80x9d published Mar. 28, 2002, a transparent underfill resin with an index of refraction less than the index of a waveguide cladding is used as an underfill material between optical devices on a flip chip and a printed wiring board. Other manufacturers may use solder bumps without the use of any underfill material to provide for unimpeded optical transmission between an optoelectronic device on the flip chip and a microlens, waveguide, or other optical element on a printed wiring board, though forfeit the attendant advantages of underfill use.
It would be beneficial to have a packaging technology for directly attaching opto-electronic flip chips to an underlying package substrate or PWB that allows secure electrical and mechanical die attach to the PWB while retaining optically unimpeded optical transmission paths between the flip-chip and the PWB. The packaging technology would allow the flip chip to be bonded effectively to a substrate, with highly reliable electrical interconnections and protective underfill material for secure die bonding, stress relief for the bumps and effective environmental protection, while retaining unrestricted free-space transmission characteristics between associated optical devices. It would be critical for electromechanical flip chips to be attached to a substrate or PWB without impeding movement of the flip chip when bonded to the PWB or package substrate.
It is an object of this invention, therefore, to provide a method for attaching bumped opto-electronic and electromechanical flip chips to a printed wiring board. It is another object of this invention to provide a flip-chip assembly with optical portions and electromechanical portions of the flip-chip assembly free from any underfill material. It is yet another object of this invention to provide a selective underfill process for bumped opto-electronic flip chips and bumped electromechanical flip chips at the die or wafer level, and to overcome other deficiencies and obstacles described above.
One aspect of the invention provides a method for attaching a flip chip to a printed wiring board. An underfill material is applied to a first portion of a bumped flip chip, maintaining an optical portion or an electromechanical portion of the flip chip free of the underfill material. The flip chip with the selective underfill is positioned on a printed wiring board, and heated to electrically and mechanically connect the flip chip to the printed wiring board while the optical portion or electromechanical portion of the flip chip remains free of the underfill material.
Another aspect of the present invention is a flip-chip assembly, including a bumped flip chip with a first portion and a second portion, and an underfill material selectively disposed on the first portion of the flip chip. The second portion of the flip chip may contain one or more optical devices or electromechanical devices. The second portion of the flip chip remains free from the underfill material when the flip chip is placed on a printed wiring board and heated to electrically connect the flip chip to the printed wiring board. The flip-chip assembly may include a printed wiring board, wherein an active surface of the flip chip is positioned and secured to the printed wiring board, with at least one opto-electronic device on the flip chip optically coupled to an associated device on the printed wiring board.
Another aspect of the invention is a selective underfill process. A bumped semiconductor wafer including at least one opto-electronic device or electromechanical device and at least one solder bump is aligned to a patterned mask. The patterned mask includes at least one barrier feature corresponding to at least one opto-electronic or electromechanical device. An underfill material is dispensed through the patterned mask onto the bumped semiconductor wafer, keeping the opto-electronic and electromechanical devices free of the underfill material. The underfill material is heated to flow the underfill material around the solder bumps, while the opto-electronic or electromechanical device remains free from the underfill material.
Another aspect of the invention is a selective underfill process based on a patterned underfill film. A bumped semiconductor wafer including at least one opto-electronic device or electromechanical device and at least one solder bump is aligned to the patterned underfill film, the patterned underfill film including a backing layer and an underfill material disposed on the backing layer with at least one open feature in the underfill material corresponding to the opto-electronic or electromechanical device. The patterned underfill film is laminated to the bumped semiconductor wafer, the backing layer is removed, and the underfill material is heated to flow the material around the at least one solder bump.